Alexander <[EMAIL PROTECTED]> writes: | Hi... | | Well, that explains a lot, thanks. | | I'm not running RAID, this is just a single UDMA/33 disk. But... what | about that extra bandwidth helping with transferring data from the HDD's | own cache?
Yes, the extra bandwidth can help there. But, for the majority of drives that cache is less than 1MB, and maybe 4MB if you have an A/V disk. Not a whole lot, but significant if you're working with lots of small files. Also, I'm pretty sure hdparam trys to defeat the cache on the disk. | And I have just heard mention of solid-state HDDs. What are those? They're just huge banks of RAM made to look like a disk. They're specialty items and you won't see a lot of them, if any. Plus, with the performance of disks getting into the 20MB/s range they're probably even rarer these days. On the other hand RAM prices are so low...Who knows? I've never seen one myself but have seen them advertised in high-end hardware magazines. They are generally VERY expensive. Gary | On 20 Jul 1998, Gary L. Hennigan wrote: | > Alexander <[EMAIL PROTECTED]> writes: | > | Hi... | > | | > | Umm: | > | | > | /dev/hda: | > | Timing buffer-cache reads: 64 MB in 1.61 seconds =39.75 MB/sec | > | Timing buffered disk reads: 32 MB in 6.87 seconds = 4.66 MB/sec | > | | > | Buffer-cache reads? Uh... explain that to me please, this particular UDMA | > | can't go past 33 MB/s. | > | > That's just the cache Linux reserves in your RAM. That's why it's | > 39.75MB/s. The fastest single disk in existence is rated at about | > 20MB/s. So the disk you give results for above is getting roughly | > 4.66MB/s and you're getting about 40MB/s from the RAM cache. | > | > | But I do believe I heard of a UDMA/66 or something like that. I'm not | > | using that here, though, so... | > | > Yes, I remember hearing something about that too. Plus, there's U2W | > SCSI rated at 80MB/s. Again, for a single disk access the bandwidth, | > whether it's 33 (UDMA), 40 (UW SCSI), 66 (UDMA/66) or 80MB/s (U2W | > SCSI), is overkill. The fastest disks manufactured are currently the | > 10,000RPM drives, e.g., Seagate Cheetah, and their peak performance is | > 20MB/s, and that's peak, which means probably only when | > reading/writing data on the outter tracks would you ever get that | > rate. Of course you benefit from the extra bandwidth if you have | > multiple devices on that bus, say you have UW SCSI rated at 40MB/s, | > then you can run two of those Chetah's simultaneously without degraded | > performance. | > | > My point was that if you're benchmarking a disk and you get greater | > than 20MB/s you're seeing the results of caching, either in RAM or on | > the disk itself. Or you have some special set up, e.g., solid state | > disk (they still make these?) or a RAID. | > | > Gary -- Unsubscribe? mail -s unsubscribe [EMAIL PROTECTED] < /dev/null
